Welcome back to this mini-series on the basic elements of an index.

I’m currently looking at what makes up an entry, which I described as “what this thing is + where to find it.” In my previous post I discussed main headings and subheadings, which form the first part of that equation. Today, I’m writing about the second part, “where to find it,” also known as locators.

Locators are the portion of the entry which tells readers where to find information about the main heading and subheading. They are like directions for the reader to follow. To be effective, locators need to be clear, specific, and accurate. 

There are three points that I want to make about locators.

The first is that a locator can be anything. Page numbers are usually the default locator, especially when indexing books, but other forms of locators can also be used, such as paragraph or policy numbers. The only criteria is that the locator is appropriate for the material being indexed.

My second point is that the locator needs to be clear and specific. Readers should understand how the locator relates to the text, and should be able to easily use the locator to find the desired information.

To give a few examples, page numbers are often augmented when referring to figures and tables or to footnotes and endnotes. For figures and tables, the page number may be placed in italics or bold, or have a fig. or t appended. For notes, the note number is usually appended to the page number, as in 253n43 or 265nn14-15. These allow the reader to more quickly pinpoint the information on the page.

For a cumulative index for a journal or multivolume series, the locator should probably include a volume or issue number alongside the page number, as in VII.343. For a policy document that I update every couple of years, I use unique policy numbers instead of page numbers, as in 20.2.1.2. These policy numbers both direct readers to the specific policy more quickly (especially if there are ten or more policies listed per page), and makes updating the index a whole lot easier as I don’t need to worry about the pagination shifting as policies are added, removed, and revised. 

When augmenting page numbers or using something different for a locator, it may be helpful to explain your choice in a headnote so readers understand how to interpret the locator. Page numbers, being the default, don’t need to be explained.

To give a few examples of locators:

conduction, 83, 84fig. 

convection, 85, 84t, 234n43

safety protocols, 2.1.13.4, 3.3.1.12, 3.4.3.6

thermodynamics, VI.343, IX.23

The last point I want to make is that the number of locators in an array should be reasonable for readers to search. As a general rule of thumb, no more than 6-10 undifferentiated locators. Larger arrays, with more than ten locators, should have subheadings to sort the references and make it easier for the reader to identify the relevant aspects of the subject. The concern is that searching through a long string of undifferentiated locators is time-consuming and may discourage the reader from finding what they need. Better to set the reader up for success by presenting locators in smaller chunks.

When writing an entry, the main heading and, possibly, subheadings, tell readers what the subject is, while the locator tells readers where to find information about that subject. Locators can be anything, and should be clear and specific. The number of locators should also be reasonable for readers to search. Do this, and you are setting your reader up for success.